C. neoformans is an important human pathogen. Several strains of this yeast were used for the production of biochemical genetic markers. All parent strains grew well on a minimal medium consisting of inorganic salts, glucose and a sinlge vitamin, thiamine. After treatment with a chemical mutagen, yeast cells were replicate plated from complete medium to minimal medium and to minimal medium supplemented alternatively with casamino acids, a mixture of purines and pyrimidines or a mixture of vitamins. Colonies which were capable of growth only with a specific nutritional supplement were selected for further study. Seventeen auxotrophs have been characterized thus far, and each exhibits a nutritional requirement for a unique amino acid, nucleic acid base or vitamin. Several of these mutants exhibit Mendelian segregation in outcrosses. The Cryptococcal capsule is believed to be a virulence factor. We sought acapsular mutants by mutagenesis of suspensions of wild-type cells, plating onto solid medium, replica-plating and staining of colonies with mucicarmine. Macroscopically colorless colonies were examined by the India ink method. Thus far 14 apparently stable acapsular mutants have been obtained. These were crossed to the wild type and segregation of the mutant trait was observed in colonies derived from single basidiospores. Several of the mutants exhibited Mendelian (50:50) segregation. These mutants appear to represent chromosomal mutations in genes required for production of the Cryptococcal capsule. Further genetic characterization is in progress.